r - Create a bivariate color gradient legend using lattice for an spplot overlaying polygons with alpha -

r - Create a bivariate color gradient legend using lattice for an spplot overlaying polygons with alpha -

i've created map overlaying polygons using spplot , alpha value of fill set 10/255 areas more polygons overlapping have more saturated color. polygons set 2 different colors (blue , red) based on binary variable in attribute table. thus, while color saturation depends on number of polygons overlapping, color depends on ratio of bluish , reddish classes of polygons.

there is, of course, no easy built-in legend need create 1 scratch. there nice solution in base of operations graphics found here. came not-so-good hack in ggplot based on this post kohske. similar question posted here , did best give solutions, couldn't come solid answer. need same myself, utilize r , utilize grid graphics.

this ggplot hack came with

variable_a <- 100 # max of variable variable_b <- 100 x <- melt(outer(1:variable_a, 1:variable_b)) # set info frame plot p <- ggplot(x) + theme_classic() + scale_alpha(range=c(0,0.5), guide="none") + geom_tile(aes(x=var1, y=var2, fill="variable_a", col.regions="red", alpha=var1)) + geom_tile(aes(x=var1, y=var2, fill="variable_b", col.regions="blue", alpha=var2)) + scale_x_continuous(limits = c(0, variable_a), expand = c(0, 0)) + scale_y_continuous(limits = c(0, variable_b), expand = c(0, 0)) + xlab("variable_a") + ylab("variable_b") + guides(fill=false) p

which gives this:

this doesn't work purposes 2 reasons. 1) because alpha value varies, sec color plotted (blue in case) overwhelms first 1 alpha values higher. right legend should have bluish , reddish mixed evenly along 1:1 diagonal. in addition, colors don't correspond map colors. 2) don't know how overlay ggplot object on lattice map created spplot. tried create grob using ggplotgrob(p), still couldn't figure out how add together grob spplot map.

the ideal solution create similar figure using lattice graphics. think using tiles right solution, best have alpha values remain constant , vary number of tiles plotted going left right (for red) , bottom top (for blue). thus, colors , saturation should match map (i think...).

any help much appreciated!

how mapping angle color, , alpha sum of 2 variables -- want?

d <- expand.grid(x=1:100, y=1:100) ggplot(d, aes(x, y, fill=atan(y/x), alpha=x+y)) + geom_tile() + scale_fill_gradient(high="red", low="blue")+ theme(legend.position="none", panel.background=element_blank())

r ggplot2 alpha lattice sp